This invention relates generally to the field of disc drive storage devices, and more particularly, but not by way of limitation, to improving the operational response of a disc drive servo system by reducing the time required to converge to an optimal gain for use by the servo system to control magnitude of current applied to a voice coil motor of the disc drive to effect head positional control, the rate of convergence of the gain controlled by a variable convergence factor.
Hard disc drives are commonly used as the primary data storage and retrieval devices in modern computer systems. In a typical disc drive, the data are magnetically stored on one or more discs that are rotated at a constant high speed and accessed by a rotary actuator assembly having a plurality of read/write heads that fly adjacent the surfaces of the discs. A read channel and interface circuit are provided to recover previously stored data from the discs to the host computer.
A closed loop digital servo system such as disclosed in U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention, is typically used to control the position of the heads relative to tracks on the discs. The tracks are defined from servo information that is written to the surfaces of the discs during manufacturing. The servo system of a disc drive thus utilizes the servo information in the performance of two primary operations: seeking and track following.
Seeking entails the movement of a selected head from an initial track to a destination track. For seeks of a sufficient length, a velocity-control approach is typically employed wherein the velocity of the head is repetitively determined and compared to a velocity profile which defines an optimum velocity trajectory for the head as it moves to the target track. The amount of current applied to an actuator coil varies in proportion to the velocity error, the actuator coil being part of a voice coil motor used to control the position of the head.
Track following entails the continued positioning of a selected head over a corresponding, selected track. A position-control approach is typically employed wherein the relative position of the head with respect to the center of the track is determined and compared to a desired position for the head. The resulting position error is used to control the amount of current that is applied to the actuator coil in order to maintain the head at the desired position relative to the track.
As will be recognized, modern disc drives typically employ an embedded servo scheme wherein the servo information is angularly spaced and interspersed among user data fields (or xe2x80x9csectorsxe2x80x9d) on the surfaces of the discs. However, the sampling rate of the servo information is typically insufficient to provide the gain necessary to maintain the heads within predetermined off track boundaries. Accordingly, a multi-rate observer is deployed to provide estimates of head position, velocity and bias at times when the heads are disposed over the user data fields. Thus, the servo system utilizes position information obtained from the discs to provide the observer with the input required to give estimates for controlling the movement of the heads during seeking and track following. Such observers (or xe2x80x9cestimatorsxe2x80x9d) are well known in the art and are discussed, for example, in U.S. Pat. No. 5,585,976 issued Dec. 17, 1996 to Pham, assigned to the assignee of the present invention.
A continuing trend in the disc drive industry is to provide disc drives with ever increasing data storage and transfer rate capacities. Some disc drives of the current generation have track densities greater than about 7,000 tracks per centimeter (about 18,000 tracks per inch). As track densities continue to increase, it becomes increasingly important to provide servo systems that are capable of accurately positioning the heads during seeks and track following modes of operation. Various gains of the servo system are typically selected to achieve a certain control response for the servo system; however, changes in environmental conditions and other factors tend to affect the characteristics of the servo system during operation.
While efforts in the prior art to adapt the operational characteristics of disc drive servo systems in view of changing environmental conditions have been successful, there is a continual need for improvements whereby servo system performance can be enhanced in view of further advancements in the art, such as continued increases in disc drive track densities.
The present invention is directed to an apparatus and method for optimizing servo gain in a disc drive.
In accordance with preferred embodiments, a disc drive includes a head adjacent a recording surface of a rotatable disc and a voice coil motor which is coupled to the head. A servo circuit applies current to the voice coil motor to controllably position the head with respect to the recording surface, the servo circuit comprising a servo processor which utilizes an internal gain to control a magnitude of the current.
The servo processor optimizes the internal gain by selecting an initial value of gain for the internal gain. Next, a gain convergence operation is performed which comprises repeatedly positioning the head and accumulating position error over a successive number of passes to iteratively converge the internal gain from the initial value of gain to a final value of gain which provides optimal performance by the servo circuit. The gain convergence operation utilizes a scale factor during each of the successive number of passes, with the scale factor also being iteratively converged over the successive number of passes from an initial value to a final, nominal value.
Significantly, by using a large initial value for the scale factor, the gain can be moved quickly from the initial value to near the final value; thereafter, using smaller values for the scale factor allows the gain to quickly and smoothly transition to the final value.
A new value for the scale factor is determined during each successive pass during the gain convergence operation in relation to a combination of a previous value for the scale factor during a previous pass, and a product of a convergence constant and a difference between the nominal value for the scale factor and the previous value for the scale factor. The magnitude of the convergence constant controls the rate of convergence of the scale factor from the initial value to the nominal value, and hence the rate of convergence of the gain from the initial value of gain to the final value of gain.
The final value of gain is a base gain which is further adapted to account for variations in torque capability of the voice coil motor. Preferably, the disc recording surface is divided into a number of concentric zones with each zone comprising a plurality of tracks. The servo processor proceeds to determine a torque capability factor for each zone which is inversely proportional to torque capability of the voice coil motor in each zone.
Next, the servo processor obtains a zone servo gain which is used for each zone in relation to a combination of the base servo gain and the associated torque capability factor, the zone servo gain used to control the magnitude of the current applied to the voice coil motor. Because magnetic flux density of the voice coil motor is typically lower near edges of permanent magnets of the motor and higher near intermediate portions of the magnets, the zone servo gain for zones disposed near innermost and outermost diameters of the recording surface are greater than the zone servo gain for zones disposed near intermediate portions of the recording surface.
Further, the gain convergence operation wherein the head is repetitively moved across the recording surface is preferably characterized as comprising a number of successively performed model reference seeks, which comprise short seeks (typically 100 tracks or less). Each model reference seek utilizes a reference current signal indicative of the current to be applied to the voice coil motor having a shape characterized as a modified one minus cosine waveform.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.